As is known in the prior art, antennas that transmit or receive signals directionally must be calibrated when they are mounted on platforms of any type because the platforms create measurement errors due to mechanical distortions in the antennas and spurious reflections, such as multi-path reflections, from portions of the platform on which the antenna is mounted.
In the prior art, antenna calibrations were performed in the far field. For measurements made in the far field, the distance between the test antenna and the signal transmitting source (or receiver) must be greater than 2D2/λ, where D is a characteristic dimension of the test antenna and λ is the wavelength. For antennas having significant aperture phase deviations and requiring low measurement uncertainties, the spacing required can exceed 8D2/λ. From this mathematical relation it can be seen that large distances are required for far field testing of large aperture (D), high frequency antennas. For example the testing distance required for far field pattern measurement of a 12 GHz parabolic reflector antenna with a 12 ft diameter would be 3500 ft. Because of such large distances far field tests must be conducted outdoors where the testing is constrained by weather and the transmission path is influenced by the weather. In addition, the path between a test antenna and a transmission source must be controlled to preclude variations caused by reflections from traffic entering the path.
In view of various problems, including those described in the previous paragraph, it is impractical to test large antennas in the far field. Fortunately, techniques have been developed to measure the far field pattern of an antenna under test on smaller indoor ranges under controlled conditions. This is called near field antenna testing. Near-field testing of an antenna is a technique that calculates a far field antenna pattern from multiple radio frequency measurements taken very close to either a transmitting antenna or a receiving antenna. The results of near field testing results are usually more accurate due to better control of spurious reflections and because the testing can usually be done indoors under controlled conditions. More specifically, advantages of near field antenna testing using these smaller ranges include: (a) reduction of outside interference, (b) reduction of testing time lost to poor weather conditions, (c) the ability to do classified testing, and (d) the reduction of electromagnetic transmissions into the environment. This last point is especially important with the current interest in environmental impact analysis. Unfortunately, when applied to the DF array calibration problems most compact antenna range processing solutions are much more complex than required for DF array manifold generation.
The far field antenna pattern is of vital concern to a system that uses a directional antenna. The far field pattern details the direction and shape of the antenna main lobe and its side lobes. Side lobes show where and how much signal an antenna will pick up or signals that will be transmitted outside of the main lobe, which is the direction of primary interest. Antenna designers normally try to make side lobes as small as possible.
In the prior art, for near field testing of an antenna, a test probe (a small antenna) is moved over a plane in front of the antenna under test, or over a cylindrical or spherical surface surrounding the antenna under test. The test probe antenna transmits or receives radio frequency signals to or from the antenna under test. The amplitude and phase of the received RF signal is recorded by the test probe at specific and equally spaced locations. A computer program processes this data and predicts the far field pattern for the antenna. The increasing power of digital computers has made this near field testing technique much easier and faster to use.
Four distinct methods or techniques of near field scanning have been developed in the prior art. These are the planar (plane-rectangular), plane-polar, cylindrical, and the spherical scanning methods. Each has particular advantages for certain applications, but all have serious limitations for the maximum size of antenna that can be measured. For example, the planar method is best suited for high gain antennas, does not require the antenna under test to move, and uses a test probe antenna that scans a rectangular measurement plane immediately in front of the antenna aperture. Probe scanning arrangements of this type are complex because the probe must be accurately moved in two directions (X,Y) to cover the entire measurement (scan) plane. A planar scanner is extremely expensive to fabricate as it gets larger because of the high mechanical tolerances that must be maintained to obtain high quality measurement results. Planar scanners in use now are typically less than about six meters maximum dimension, but some are larger.
Plane-polar scanners are similar in application to planar (plane-rectangular) scanners. They require the antenna to rotate about a single axis and use a probe that moves along a line perpendicular to and intersecting the antenna rotation axis. The combined motion produces a circular disc measurement surface immediately in front of the antenna aperture. This method can handle antennas larger than for the planar scanner method but requires the test antenna to be rotated. Again, as the antenna size grows, the method becomes very expensive and is not practical for extremely large antennas. The largest antenna measured in this way was twenty meters in diameter. Also, the computation of the far field is a little more difficult because the data samples are distributed on concentric rings rather than on a rectangular grid.
With the cylindrical scanning technique, the measurement plane is a cylinder surrounding the antenna under test and the cylinder oriented coaxially with the rotation axis of the antenna. The probe moves along a line parallel to the rotation axis. Computation of the far field is more complicated and time consuming than in planar scanning and, since the method requires motion of the antenna, it is limited to the measurement of modest size antennas.
The spherical scanner technique requires the antenna under test to rotate about two orthogonal axes, and uses a stationary antenna probe. The motion of the antenna relative to the stationary probe antenna produces an apparent spherical measurement surface centered at the antenna. The method is limited to fairly small antennas because of the complex antenna rotation needed. Also, the computations required to obtain the far field data are much more complicated than the planar case and may be prohibitive for large antennas.
Each of the four methods listed can not be applied to large antennas of 30–100 meters or larger in size. For planar scanning, scanner construction to maintain the required tolerance is technically and financially prohibitive. For the other methods, the machinery necessary to rotate the antenna is similarly prohibitive. In addition, the superstructures required to support and move the measurement antenna probe in the planar, plane-polar, and cylindrical methods can produce unwanted reflections that limit the accuracy of the measurement. Further, locating test probe antennas in the near field where the transmit beam of a transmitting antenna is unfocused may result in unacceptable performance, primarily due to phase errors and due to the magnification of the effects of system design tolerances. In general, near-field scanning requires extremely careful mechanical alignment and the problems of maintaining this alignment are increased with size of an antenna under test.
In most spherical near field antenna testing two orthogonal axes describe positioning of a test probe antenna over a spherical surface surrounding and centered on the antenna under test. The axes are azimuth phi (φ), and elevation which is zenith minus theta (θ) (measured from the vertical). For high accuracy the two axes must be close to 90 degrees from each other, the probe must be at a constant radius on a sphere from the antenna under test, and the probe position must be accurately known. In most spherical near-field antenna testing an antenna under test is rotated on a turntable while a test probe is only moved up and down along the spherical surface to cause an effective probe movement over the spherical surface at a fixed elevation.
U.S. Pat. No. 5,039,991 issued Aug. 13, 1991 to Boeing describes a test system for processing antenna outputs from scaled direction finding (DF) antennas, on a scaled aircraft test platform, with a scaled RF signal. The DF antennas developed and tested by the system disclosed in this patent are to be mounted on an aircraft and to accurately determine the direction of a source of received electromagnetic radiation. The aircraft platform itself perturbs the phase and amplitude of received electromagnetic radiation and this is accounted for by the calibration process.
A correlation device is used for correlating signals received by a plurality of actual DF antennas on the actual aircraft with data that is empirically derived using the scaled test model and stored in a database representative of the performance of DF system (DF antennas and aircraft). The amplitude and phase characteristics of transmissions received from a signal source are correlated to the stored data and the azimuth and elevation angle of the signal source is determined from the stored data that most nearly correlates to the measured amplitude and phase generated complex vectors.
The most elemental form of DF system determines the azimuth of RF transmissions received by the antenna system in a single horizontal plane. In addition to determining azimuth information, the elevation of received RF transmissions in a vertical plane and range data of the received RF transmissions can also be determined.
The spacing of antenna elements on an aircraft to be modeled is typically established as a function of the wavelength of the RF transmissions to be received. Thus, to allow the use of an antenna model that is physically smaller than the actual antenna, the RF transmissions used during modeling must have a wavelength that is proportionally smaller than the wavelength of the transmissions to be received by the actual antenna. For example, if the model antenna is one-tenth the size of the actual antenna, and the aircraft platform model is one-tenth the size of the actual aircraft, the frequency of the RF transmissions used to evaluate the model's performance must be ten times that of the RF transmissions to be received by the actual antenna and aircraft.
The prior art modeling described in the previous paragraphs for U.S. Pat. No. 5,039,991 scales the values of the parameters in the equation 2D2/λ, which defines the far field, so that model antenna and model platform testing data gathered using scaled frequencies is actually gathered in the far field as defined by the equation. While this redefined far field distance is shorter than when testing with full size antennas and platform, such as an aircraft, it is still relatively large when compared to gathering model data in the near field (less than 2D2/λ), and then converting the near field data to the far field for use in correlation of signals received by DF antennas on an actual platform.
An earlier paper by N. Saucier and K. Struckman, Direction Finding Using Correlation Techniques, IEEE Antenna Propagation Society International Symposium, pp. 260–263, June 1975, teaches the same thing as taught in U.S. Pat. No. 5,039,991.
U.S. Pat. No. 5,721,554, issued on Feb. 24, 1998 to Stanley R. Hall et al., describes a near field planar wave front generation method that uses a relatively small number (three to five) of transmitting antennas to create a synthesized one-dimensional linear plane of radiation over 10 to 20 wavelengths at a specific location on an antenna array of a system under test at a specific frequency and distance, typically in the range of from 100 to 200 feet. Hall et al. do not suggest a down range returns simulator according to the claimed invention.
Thus, there remains a need in the prior art for a simpler, more accurate way of performing near field testing of antennas on a compact antenna range of a size less than one-hundred feet, and more particularly of spherical, near field, antenna testing, and then using the test data to create a calibration table that is used to correct actual measurements in the far field.
There also remains a need in the art for a technique for transforming near field measurement data into an accurate far field calibration table for correcting direction finding and other antenna type outputs.